Solar thermal heating systems (STHS) utilise energy from the sun, in the form of light and heat, to supply heat to hot water systems. This is achieved by using a solar collector filled with a liquid medium, which absorbs heat from the radiation of the sun (it does not have to be sunny to work) and transfers this heat, via a heat exchange system to a dual coil hot water tank. Collector technology is split between flat plate collectors and evacuated tube systems. The former are cheaper per m² than the latter, but are not as efficient. STHS will provide a portion of the annual domestic hot water requirements depending on the details of the end use and size of solar collector system installed, and is generally around 50% of hot water requirements over the course of a year, although in reality this is often higher. This will be around 95% of hot water requirements from the end of May through to mid-September, whilst in winter a 20% contribution can be expected on average.
| Figure 9 Solar thermal slates |
There are now a number of solar thermal tile products entering the market and these allow for total roof integration options, although, like PV, the product is still only available in a traditional grey/blue slate type colouring and is more expensive than both flat-plate and evacuated tube systems.
The hot water from a solar thermal system is normally supplied as a pre-heat to a twin coil hot water storage tank linked to the main heating system for the home. There will also be a drain-back unit (on some systems), pumps, controllers and pipe-work to interface with the existing boiler system.
Both flat-plate and evacuated tube collectors tend to be guaranteed for around 20 years but are known to last well over 30 years. Other parts such as pumps have standard time guarantees associated with standard plumbing products. Annual maintenance requirements are minimal and it would be reasonable to check the system on a 5 year cycle, although it could be included with standard annual boiler servicing.
STHS collectors are best located on un-shaded, pitched roofs, facing due south for optimum performance, although other orientations within 45 degrees either direction from due south or horizontal mounting do not significantly affect output. Even at east/west facing alignments, the system will still perform to around 75% of efficiency.